Trivalent chromium plating formulations and processes

ABSTRACT

An electrolyte solution for chrome plating from trivalent chromium is prepared by dissolving in an aqueous medium a trivalent chromium salt (e.g., chromium (III) chloride or chromium (III) sulfate), dissolving an oxalate compound (e.g., sodium oxalate, potassium oxalate, or oxalic acid), dissolving a metal salt (e.g., aluminum sulfate or aluminum chloride), dissolving an alkali metal sulfate (e.g., sodium sulfate or potassium sulfate), and dissolving an alkali metal halide (e.g., sodium fluoride or potassium fluoride). A substrate is chrome plated from trivalent chromium using the electrolyte solution by passing a current between a cathode and an anode through the electrolyte solution to deposit chromium on the substrate.

BACKGROUND 1. Technical Field

The present disclosure relates to chrome plating and, more particularly,to using trivalent chromium for plating a substrate with chromium.

2. Related Art

Chrome plating is an electroplating process that provides a chromecoating on a substrate. Hard chrome plating provides a chrome coatinghaving a thickness typically about 10 microns or greater, therebyproviding hardness and wear resistance to the coated substrate. Theother type of chrome plating is decorative chrome plating, whichprovides a chrome coating having a thickness typically ranging fromabout 0.1 to about 0.5 microns. Chrome plating is often performed usingbaths containing chromic acid and catalysts based on fluorides, sulfatesor organic acids. Chromic acid has chromium in its hexavalent form,chromium (VI), which is highly toxic and a carcinogen.

There is a need for improved chrome plating methods and formulations ofsolutions used in chrome plating.

SUMMARY

In accordance with embodiments of the present disclosure, variousmethods and formulations are provided for chrome plating a substrateusing a trivalent chromium solution that does not include boric acid,while still resulting in a chromium layer (e.g., a chromium coating)formed on the substrate that may be structurally robust and reliable,yet cost-effective. Thus, the methods and formulations described hereinmay advantageously be used for hard chrome plating to form hard chromiumlayers (e.g., a robust, functional chromium layer of greater than 10microns). However, the present disclosure is not limited to hard chromeplating and the methods and formulations described herein may also beadvantageously used to effectively and efficiently perform decorativechrome plating, which forms decorative chromium layers (e.g., a chromiumlayer ranging from 0.25 micron to 1.0 micron).

In one example embodiment, a method of preparing an electrolyte solutionfor chrome plating includes dissolving in an aqueous medium a trivalentchromium salt in an amount ranging from about 0.1 mol to about 0.9 molper liter of the electrolyte solution, dissolving an oxalate compound inan amount ranging from about 0.1 mol to about 3.0 mol per liter of theelectrolyte solution, and dissolving a metal salt in an amount rangingfrom about 0.1 mol to about 4.0 mol per liter of the electrolytesolution, an alkali metal sulfate in an amount ranging from about 0.1mol to about 2.0 mol per liter of the electrolyte solution, and analkali metal halide in an amount ranging from about 0.1 mol to about 0.5mol per liter of the electrolyte solution per liter of the electrolytesolution. The step of dissolving the trivalent chromium salt, theoxalate compound, the metal salt, the alkali metal sulfate, and thealkali metal halide may be performed in the following order: (1)dissolving the trivalent chromium salt, (2) dissolving the oxalatecompound, (3) dissolving the metal salt, (4) dissolving the alkali metalsulfate, and (5) dissolving the alkali metal halide. The order of steps(1) and (2) may be reversed or be performed concurrently.

The trivalent chromium salt may include chromium (III) chloride and/orchromium (III) sulfate. The oxalate compound may include sodium oxalate,potassium oxalate, and/or oxalic acid. The metal salt may includealuminum sulfate and/or aluminum chloride. The alkali metal sulfate mayinclude sodium sulfate and/or potassium sulfate. The alkali metal halidemay include sodium fluoride and/or potassium fluoride.

The step of dissolving the oxalate compound may include stirring theoxalate compound at a temperature ranging from about 70° C. to about 80°C. for a time ranging from about 1 hour to about 3 hours. The method mayfurther include adjusting the pH of the electrolyte solution to a pHranging from about 2 to about 4.

The method may further include adding sodium lauryl sulfate and/orpotassium lauryl sulfate in an amount ranging from about 0.1 g to about1 g per liter of the electrolyte solution. The method may furtherinclude adding sodium bromide and/or potassium bromide in an amountranging from about 0.1 g to about 1 g per liter of the electrolytesolution.

In an additional example embodiment, a method for chrome plating asubstrate includes preparing an electrolyte solution by dissolving, atrivalent chromium salt, an oxalate compound, aluminum sulfate, alkalimetal sulfate, and alkali metal fluoride; passing a current between acathode and an anode through the electrolyte solution to depositchromium on the substrate; and maintaining the electrolyte solution at apH ranging from about 2 to about 4. The step of preparing theelectrolyte solution may include dissolving the trivalent chromium saltin an amount ranging from about 0.1 mol to about 0.9 mol per liter ofthe electrolyte solution, dissolving the oxalate compound in an amountranging from about 0.1 mol to about 3.0 mol per liter of the electrolytesolution, and dissolving the metal salt in an amount ranging from about0.1 mol to about 4.0 mol per liter of the electrolyte solution, analkali metal sulfate in an amount ranging from about 0.1 mol to about2.0 mol per liter of the electrolyte solution, and an alkali metalhalide in an amount ranging from about 0.1 mol to about 0.5 mol perliter of the electrolyte solution. The method may further includemaintaining the electrolyte solution at a temperature ranging from about30° C. to about 40° C. during the step of passing the current.

The step of passing the current may be performed using an anodeincluding a carbonaceous electrode material, such as a graphite anode.The step of passing the current may include applying a current densityranging from about 10 A/dm² to about 30 A/dm², The step of the passingthe current may include applying a pulsed current having a duty cycleranging from about 20% to about 80%.

The step of passing the current may be performed until a chromium layerhaving a thickness greater than about 5 microns and hardness greaterthan about 800 HV is formed on the substrate. The step of passing thecurrent to deposit chromium on the substrate may include passing thecurrent to deposit chromium on a steel substrate, a copper substrate, anickel substrate, a copper-coated substrate, or a nickel-coatedsubstrate. The method may further include depositing, responsive to thestep of passing the current, chromium on the substrate or co-depositingchromium and carbon on the substrate.

In another example embodiment, a method of preparing an electrolytesolution for chrome plating includes performing the following steps inorder: (1) providing trivalent chromium by dissolving a trivalentchromium salt, (2) forming complexes of oxalate and trivalent chromiumby dissolving an oxalate compound, (3) buffering the electrolytesolution by dissolving a metal salt, (4) increasing the conductivity bydissolving an alkali metal sulfate, and (5) increasing the wettingproperty of the electrolyte solution by dissolving alkali metal halide.The order of steps (1) and (2) may be reversed or be performedconcurrently.

In yet another example embodiment, an electrolyte solution is preparedby one of the methods described above. For example, an electrolytesolution includes, per liter of the electrolyte solution, a trivalentchromium salt in an amount ranging from about 0.1 mol to about 0.9 mol,an oxalate compound in an amount ranging from about 0.1 mol to about 3.0mol, a metal salt in an amount ranging from about 0.1 mol to about 4.0mol, an alkali metal sulfate in an amount ranging from about 0.1 mol toabout 2.0 mol, and an alkali metal halide in an amount ranging fromabout 0.1 mol to about 0.5 mol.

The scope of the invention is defined by the claims, which areincorporated into this section by reference. A better understanding ofthe methods and formulations for chrome plating of the presentdisclosure, as well as an appreciation of the above and additionaladvantages thereof, will be afforded to those of skill in the art by aconsideration of the following detailed description of one or moreexample embodiments thereof. In this description, reference is made tothe various views of the appended sheets of drawings, which are brieflydescribed below, and within which, like reference numerals are used toidentify like ones of the elements illustrated therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example process for preparing a trivalent chromiumelectrolyte solution in accordance with an embodiment of the presentdisclosure.

FIG. 2 is an image of example solutions formed during the process ofFIG. 1.

FIG. 3 illustrates an example process for chrome plating in accordancewith an embodiment of the present disclosure.

FIG. 4 is an image of chrome plated substrates formed by the process ofFIG. 3, each plated at a different pH.

FIG. 5 is an image of chrome plated substrates formed by the process ofFIG. 3, each plated at a different temperature.

FIG. 6 is an image of chrome plated substrates formed by the process ofFIG. 3, each plated at a different current density using direct currentplating.

FIG. 7 is an image of chrome plated substrates formed by the process ofFIG. 3, each plated at a different average current density using pulsedcurrent plating.

FIG. 8 is a graph showing thickness of chromium layers formed by theprocess of FIG. 3 using pulsed current plating at different pulsefrequencies and duty cycles.

FIG. 9 is an image of a chrome plated substrate formed by the process ofFIG. 3 using pulsed current.

FIG. 10 is an image of a chrome plated substrate formed by the processof FIG. 3 using direct current.

FIG. 11 is a scanning electron microscopy (SEM) image of a cross-sectionof the chrome plated substrate of FIG. 9.

FIG. 12 is an SEM image of a cross-section of the chrome platedsubstrate of FIG. 10.

FIG. 13 is an image of a chrome plated substrate formed by the processof FIG. 3 using an electrolyte solution prepared by dissolving chromium(III) sulfate and an oxalate compound but not dissolving an alkali metalsulfate.

FIG. 14 is an image of a chrome plated substrate formed by the processof FIG. 3 using the electrolyte solution prepared by the process of FIG.1 without the step of dissolving the surfactant.

FIG. 15 is an image of a chrome plated substrate formed by the processof FIG. 3 using the electrolyte solution prepared by the process of FIG.1 including the step of dissolving the surfactant.

FIGS. 16A-B are SEM images of a part of the chrome plated substrate ofFIG. 14.

FIGS. 17A-B are SEM images of a part of the chrome plated substrate ofFIG. 15.

FIG. 18 is an SEM image of chrome deposits on a chrome plated substrateformed by the process of FIG. 3.

FIG. 19 is an image of chrome plated substrates formed by the process ofFIG. 3 that have been bent to show resilience of chromium layers tobending.

FIG. 20 is an image of a chrome plated substrate formed by the processof FIG. 3 on which abrasion testing has been performed to determine wearresistance of a chromium layer.

DETAILED DESCRIPTION

FIG. 1 illustrates an example process 100 for preparing a trivalentchromium electrolyte solution (also referred to as a trivalent chromiumplating formulation). The compound of the first block is dissolved in anaqueous medium such as water, and a respective compound of eachsubsequent block is dissolved in the solution resulting from theprevious block.

At block 102, a trivalent chromium salt is dissolved. The trivalentchromium salt is a trivalent chromium source. In one or moreembodiments, trivalent chromium salt includes a chromium (III) halide,chromium (III) sulfate (e.g., Cr₂(SO₄)₃, Cr₂(SO₄)₃.12H₂O, and/or otherchromium (III) sulfates), and/or other chromium (III) salts. Thechromium (III) halide may include, for example, chromium (III) chloride(e.g., CrCl₃, CrCl₃.5H₂O, CrCl₃.6H₂O, and/or other chromium (III)chlorides). The amount of the trivalent chromium salt that is dissolvedmay range from about 0.1 mol (moles) to about 0.9 mol per liter of theelectrolyte solution to be formed. The amount of the trivalent chromiumsalt that is dissolved may be about 0.1 mol, 0.2 mol, 0.3 mol, 0.4 mol,0.5 mol, 0.6 mol, 0.7 mol, 0.8 mol, or 0.9 mol per liter of theelectrolyte solution, where any value may form an upper end point or alower end point, as appropriate.

The trivalent chromium salt may be dissolved by stirring for 15 minutesat ambient temperature, at room temperature, at about 25° C., or at atemperature ranging from about 20° C. to about 30° C. The stirring maybe performed for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25minutes, or 30 minutes, where any value may form an upper end point or alower end point, as appropriate, or until all the trivalent chromiumsalt has been dissolved. The temperature at which block 102 is performedmay be about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40° C.,where any value may form an upper end point or a lower end point, asappropriate.

At block 104, an oxalate compound is dissolved. The oxalate compoundincludes oxalate, which may function as a complexing agent. In one ormore embodiments, the oxalate compound includes an alkali metal oxalate(e.g., sodium oxalate (Na₂C₂O₄), potassium oxalate (K₂C₂O₄), and/orother alkali metal oxalates) and/or an acid of oxalate (e.g., oxalicacid (H₂C₂O₄) and/or other acids of oxalate). The amount of the oxalatecompound that is dissolved may range from about 0.1 mol to about 3.0 molper liter of the electrolyte solution to be formed. The amount of theoxalate compound that is dissolved may be about 0.1 mol, 0.2 mol, 0.4mol, 0.6 mol, 0.8 mol, 1.0 mol, 1.2 mol, 1.4 mol, 1.6 mol, 1.8 mol, 2.0mol, 2.2 mol, 2.4 mol, 2.6 mol, 2.8 mol, or 3.0 mol per liter of theelectrolyte solution, where any value may form an upper end point or alower end point, as appropriate.

To dissolve the oxalate compound and form a complex of oxalate andtrivalent chromium, the oxalate compound may be put in solution (e.g.,the solution resulting from block 102 or another block performed priorto block 104), the solution may be heated to a higher temperatureranging from about 70° C. to about 80° C., and the solution may bestirred for about 1 hour to about 3 hours. The solution may then becooled (e.g., to ambient temperature, to room temperature, to about 25°C., or to a temperature ranging from about 20° C. to about 30° C.).Alternatively, the oxalate compound may be dissolved without heating, inwhich case a complex of oxalate and trivalent chromium is formed in 3 to4 days. Advantageously, heating the solution to a temperature rangingfrom about 70° C. to about 80° C. at block 104 allows the electrolytesolution to be prepared more quickly. Accordingly, the stirring may beperformed for about 1 hour, 1 hour and 15 minutes, 1 hour and 30minutes, 1 hour and 45 minutes, 2 hours, 2 hours and 15 minutes, 2 hoursand 30 minutes, 2 hours and 45 minutes, 2 hours and 45 minutes, or 3hours, where any value may form an upper end point or a lower end point,as appropriate. Further, the temperature at which block 104 is performedmay be at about 50° C., 55° C., 60° C., 65° C., 70° C., 75° C., 80° C.,85° C., or 90° C., where any value may form an upper end point or alower end point, as appropriate.

At block 106, a metal salt is dissolved. The metal salt is a metal ionsource that dissolves to provide metal ions such as aluminum ions, whichmay function as a buffer and may provide ionic strength due to the highvalence of the metal ion in solution (e.g., Al³⁺). In one or moreembodiments, the metal salt includes a group 13 metal salt such as analuminum salt (e.g., aluminum sulfate (Al₂(SO₄)₃), an aluminum halidesuch as aluminum chloride (AlCl₃), and/or other aluminum salts) and/orother metal salts. The amount of the metal salt may range from about 0.1mol to about 4.0 mol per liter of the electrolyte solution to be formed.The amount of the metal salt that is dissolved may be about 0.1 mol, 0.2mol, 0.4 mol, 0.6 mol, 0.8 mol, 1.0 mol, 1.2 mol, 1.4 mol, 1.6 mol, 1.8mol, 2.0 mol, 2.2 mol, 2.4 mol, 2.6 mol, 2.8 mol, 3.0 mol, 3.2 mol, 3.4mol, 3.6 mol, 3.8 mol, or 4.0 mol per liter of the electrolyte solution,where any value may form an upper end point or a lower end point, asappropriate.

The metal salt may be dissolved by stirring for 15 minutes at ambienttemperature, at room temperature, at about 25° C., or at a temperatureranging from about 20° C. to about 30° C. The stirring may be performedfor about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or30 minutes, where any value may form an upper end point or a lower endpoint, as appropriate, or until all the metal salt has been dissolved.The temperature at which block 106 is performed may be about 10° C., 15°C., 20° C., 25° C., 30° C., 35° C., or 40° C., where any value may forman upper end point or a lower end point, as appropriate.

At block 108, an alkali metal salt is dissolved. The alkali metal saltmay increase the conductivity of the electrolyte solution. In one ormore embodiments, the alkali metal salt includes an alkali metal sulfate(e.g., sodium sulfate (Na₂SO₄), potassium sulfate (K₂SO₄), and/or otheralkali metal sulfates). The amount of the alkali metal sulfate that isdissolved may range from about 0.1 mol to about 2.0 mol of theelectrolyte solution to be formed. The amount of the alkali metalsulfate that is dissolved may be about 0.1 mol, 0.2 mol, 0.3 mol, 0.4mol, 0.5 mol, 0.6 mol, 0.7 mol, 0.8 mol, 0.9 mol, 1.0 mol, 1.1 mol, 1.2mol, 1.3 mol, 1.4 mol, 1.5 mol, 1.6 mol, 1.7 mol, 1.8 mol, 1.9 mol, or2.0 mol per liter of the electrolyte solution, where any value may forman upper end point or a lower end point, as appropriate.

The alkali metal sulfate may be dissolved by stirring for 15 minutes atambient temperature, at room temperature, at about 25° C., or at atemperature ranging from about 20° C. to about 30° C. The stirring maybe performed for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25minutes, or 30 minutes, where any value may form an upper end point or alower end point, as appropriate, or until all the metal salt has beendissolved. The temperature at which block 106 is performed may be about10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40° C., where anyvalue may form an upper end point or a lower end point, as appropriate.

At block 110, an alkali metal halide is dissolved. The alkali metalhalide may provide the electrolyte solution with wetting and etchingproperties, and may help chromium adhesion during chrome plating. In oneor more embodiments, the alkali metal halide includes an alkali metalfluoride (e.g., sodium fluoride (NaF), potassium fluoride (KF), and/orother alkali metal fluorides) and/or other alkali metal halides. Theamount of the alkali metal halide that is dissolved may range from about0.1 mol to about 0.5 mol per liter of the electrolyte solution to beformed. The amount of the alkali metal halide that is dissolved may beabout 0.10 mol, 0.15 mol, 0.20 mol, 0.25 mol, 0.30 mol, 0.35 mol, 0.40mol, 0.45 mol, or 0.50 mol per liter of the electrolyte solution, whereany value may form an upper end point or a lower end point, asappropriate.

The alkali metal halide may be dissolved by stirring for 15 minutes atambient temperature, at room temperature, at about 25° C., or at atemperature ranging from about 20° C. to about 30° C. The stirring maybe performed for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25minutes, or 30 minutes, where any value may form an upper end point or alower end point, as appropriate, or until all the alkali metal halidehas been dissolved. The temperature at which block 106 is performed maybe about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40° C.,where any value may form an upper end point or a lower end point, asappropriate.

At block 112, a surfactant may be dissolved. The surfactant may preventor reduce pitting and reduce gas generation (e.g., chlorine gas,hydrogen gas, etc.) during chrome plating. In some embodiments, thesurfactant includes sodium lauryl sulfate (NaC₁₂H₂₅SO₄), potassiumlauryl sulfate (KC₁₂H₂₅SO₄), and/or other surfactants. The amount of thesurfactant may range from about 0.0001 mol to 0.01 mol per liter of theelectrolyte solution to be formed. The amount of the surfactant that isdissolved may be about 0.0001 mol, 0.0002 mol, 0.0004 mol, 0.0006 mol,0.0008 mol, 0.0010 mol, 0.0020 mol, 0.0040 mol, 0.0060 mol, 0.0080 mol,or 0.0100 mol per liter of the electrolyte solution, where any value mayform an upper end point or a lower end point, as appropriate. Forexample, the amount of sodium lauryl sulfate or potassium lauryl sulfatemay range from about 0.1 g to about 1 g per liter of the electrolytesolution to be formed.

At block 114, an alkali metal halide (e.g. alkali metal bromide) isdissolved. The alkali metal bromide may reduce the generation of gas(e.g., chlorine gas, hydrogen gas, etc.) during chrome plating. In someembodiments, the alkali metal bromide includes sodium bromide (NaBr),potassium bromide (KBr), or other alkali metal bromides. The amount ofthe surfactant may range from about 0.001 mol to 0.05 mol per liter ofthe electrolyte solution to be formed. The amount of the alkali metalbromide that is dissolved may be about 0.001 mol, 0.002 mol, 0.004 mol,0.006 mol, 0.008 mol, 0.010 mol, 0.020 mol, 0.030 mol, 0.040 mol, or0.050 mol per liter of the electrolyte solution, where any value mayform an upper end point or a lower end point, as appropriate. Forexample, the amount of sodium bromide or potassium bromide may rangefrom about 0.1 g to about 1 g per liter of the electrolyte solution tobe formed.

At block 116, the pH may be adjusted. In some embodiments, the pH isadjusted using one or more acids or bases, such as potassium hydroxide(KOH), sodium hydroxide (NaOH), and/or sulfuric acid (H₂SO₄). The pH ofthe electrolyte solution may be adjusted to a range from about 2 toabout 4. The pH may be adjusted to about 2, 2.2, 2.4, 2.6, 2.8, 3.0,3.2, 3.4, 3.6, 3.8, or 4.0, where any value may form an upper end pointor a lower end point, as appropriate.

At block 118, time may be provided to reach equilibrium state. In someembodiments, the solution is left to stand for a time ranging from 1hour to 2 days to reach the equilibrium state. The time provided toreach the equilibrium state may be about 1 hour, 3 hours, 6 hours, 9hours, 12 hours, 15 hours, 18 hours, 21 hours, 24 hours, 27 hours, 30hours, 33 hours, 36 hours, 39 hours, 42 hours, 45 hours, or 48 hours,where any value may form an upper end point or a lower end point, asappropriate.

In some embodiments, process 100 is performed in the order presented. Inother embodiments, process 100 is performed in a different order. Someblocks may be performed in order while other blocks are performed in adifferent order. For example, blocks 102, 104, and 106 may be performedin order, while blocks 108, 110, 112, 114, 116, and 118 may be performedin a different order after blocks 102, 104, and 106. In another example,blocks 102, 104, 106, 108, and 110 may be performed in order, whileblocks 112, 114, 116, and 118 may be performed in a different order. Agroup of blocks may be performed before another group of blocks. Forexample, blocks 102, 104, and 106 may be performed in any order, andafter blocks 102, 104, and 106 are performed, blocks 106, 108, and 110may be performed in any order. Other orders are contemplated as oneskilled in the art will appreciate. Further, one or more of blocks 112,114, 116, and 118 may be omitted in some embodiments.

Example 1

In an example of performing blocks 102 to 110, chromium (III) chloridein the amount of about 159 g (about 0.6 mol) per liter of electrolytesolution to be formed is dissolved in water, which results in a darkgreen solution. Although chromium (III) chloride was used in thisexample, one or more other chromium (III) salts (e.g., one or more otherchromium (III) halides and/or chromium (III) sulfate) may be usedinstead of, or in addition to, chromium (III) chloride. A solution 202shown in FIG. 2 illustrates the dark green solution diluted 10 times forgood color contrast. Then, sodium oxalate in the amount of about 80.4grams (about 0.6 mol) per liter of the electrolyte solution to be formedis dissolved in the dark green solution, which results in a darkgrey-purple solution. Although sodium oxalate was used in this example,one or more other oxalate compounds (e.g., one or more other alkalimetal oxalates and/or one or more acid of oxalate) may be used insteadof, or in addition to, sodium oxalate. A solution 204 shown in FIG. 2illustrates the dark grey-purple solution diluted 10 times for goodcolor contrast. The color change from dark green to dark grey-purple mayindicate the formation of the complex of trivalent chromium and oxalate.Then, aluminum sulfate in the amount of about 126.1 grams (about 0.2mol), sodium sulfate in the amount of about 184.6 grams (about 1.3 mol),and sodium fluoride in the amount of about 16.8 grams (0.4 mol) perliter of the electrolyte solution to be formed is dissolved in the darkgrey-purple solution, which forms a dark grey-green solution, which maybe the final electrolyte solution for use in chrome plating. Althoughaluminum sulfate was used in this example, one or more other metal salts(e.g., one or more other aluminum salts) may be used instead of, or inaddition to, aluminum sulfate. Also, although sodium sulfate was used inthis example, one or more other alkali metal salts (e.g., one or moreother alkali metal sulfates) may be used instead of, or in addition to,sodium sulfate. Further, although sodium fluoride was used in thisexample, one or more other alkali metal halides (e.g., one or more otheralkali metal fluorides) may be used instead of, or in addition to,sodium fluoride. A solution 206 shown in FIG. 2 illustrates the darkgrey-green solution diluted 10 times for good color contrast. Theelectrolyte solution may be left to stand for about 1 day to reach anequilibrium state. The resulting electrolyte solution may have atrivalent chromium concentration of about 0.6 M (moles/L), a chlorideconcentration of about 1.8 M, an oxalate concentration of about 0.6 M,an aluminum concentration of about 0.4 M, a sodium concentration ofabout 4.2 M, and a sulfate concentration of about 1.9 M.

Example 2

In another example of performing blocks 102 to 110, chromium (III)sulfate in the amount of about 235 g (about 0.6 mol) per liter ofelectrolyte solution to be formed is dissolved in water. Althoughchromium (III) sulfate was used in this example, one or more otherchromium (III) salts (e.g., one or more chromium (III) halides) may beused instead of, or in addition to, chromium (III) chloride. Then,sodium oxalate in the amount of about 80.4 grams (about 0.6 mol) perliter of the electrolyte solution to be formed is dissolved. Althoughsodium oxalate was used in this example, one or more other oxalatecompounds (e.g., one or more other alkali metal oxalates and/or one ormore acid of oxalate) may be used instead of, or in addition to, sodiumoxalate. Then, aluminum sulfate in the amount of about 126.1 grams(about 0.2 mol), sodium sulfate in the amount of about 184.6 grams(about 1.3 mol), and sodium fluoride in the amount of about 16.8 grams(0.4 mol) per liter of the electrolyte solution to be formed isdissolved. Although aluminum sulfate was used in this example, one ormore other metal salts (e.g., one or more other aluminum salts) may beused instead of, or in addition to, aluminum sulfate. Also, althoughsodium sulfate was used in this example, one or more other alkali metalsalts (e.g., one or more other alkali metal sulfates) may be usedinstead of, or in addition to, sodium sulfate. Further, although sodiumfluoride was used in this example, one or more other alkali metalhalides (e.g., one or more other alkali metal fluorides) may be usedinstead of, or in addition to, sodium fluoride. The electrolyte solutionmay be left to stand for about 1 day to reach an equilibrium state. Theresulting electrolyte solution may have a trivalent chromiumconcentration of about 1.2 M, an oxalate concentration of about 0.6 M,an aluminum concentration of about 0.4 M, a sodium concentration ofabout 4.2 M, and a sulfate concentration of about 3.7 M.

FIG. 3 illustrates an example process 300 for chrome plating, At block302, an electrolyte solution is prepared, such as by process 100 ofFIG. 1. At block 304, a cathode and an anode are placed in theelectrolyte solution, the cathode including the substrate, and a currentis passed between the cathode and the anode through the electrolytesolution to deposit chromium on the substrate. The substrate may be asteel substrate, a copper substrate, a nickel substrate, a copper-coatedsubstrate, or a nickel-coated substrate. However, other substrates arecontemplated as one skilled in the art will appreciate.

The anode may include a carbonaceous electrode material. For example,the carbonaceous anode may be a graphite anode or other anode thatincludes carbon. The graphite anode may be used for chloride-basedelectrolyte solutions (e.g., electrolyte solutions that include one ormore compounds with chloride such as chromium (III) chloride),sulfate-based electrolyte solutions (e.g., electrolyte solutions thatinclude one or more compounds with sulfate such as chromium (III)sulfate), or chloride and sulfate-based electrolyte solutions (e.g.,electrolyte solutions that include one or more compounds with chlorideand one or more other compounds with sulfate). Advantageously, thegraphite anode or other carbonaceous anode minimizes gas evolution andformation of undesirable byproducts, as well as facilitating a desirabledeposition rate (e.g., ranging from about 1 microns to about 2 micronsper minute). Alternatively, a platinum anode or a platinized titaniumanode may be used for sulfate-based electrolyte solutions (e.g.,electrolyte solutions that include one or more compounds with sulfatesuch as chromium (III) sulfate) or chloride and sulfate-basedelectrolyte solutions (e.g., electrolyte solutions that include one ormore compounds with chloride and one or more other compounds withsulfate). For example, the platinum anode or platinized titanium anodemay be used when the electrolyte solution does not include compoundswith chloride such that chlorine gas is not produced, or when theelectrolyte solution has less chloride such that less chlorine gas isgenerated (e.g., there is no need to reduce the generation of chlorinegas using a carbonaceous anode).

In some embodiments, direct current is used. The direct current mayprovide a current density ranging from about 5 A/dm² to about 50 A/dm².The value of the current density may be adjusted depending on theseparation between the cathode and anode. The current density may beabout 5 A/dm², 10 A/dm², 15 A/dm², 20 A/dm², 25 A/dm², 30 A/dm², 35A/dm², 40 A/dm², 45 A/dm², or 50 A/dm², where any value may form anupper end point or a lower end point, as appropriate, depending on theseparation between the cathode and anode. For example, a current densityranging from about 10 A/dm² to about 30 A/dm² may be applied when thecathode and the anode is separated by about 3 cm.

In other embodiments, pulsed current is used. The pulsed current mayprovide an average current density ranging from about 5 A/dm² to about50 A/dm². The value of the average current density may be adjusteddepending on the separation between the cathode and anode. The peakcurrent density may be twice of the average current density. The averagecurrent density may be about 5 A/dm², 10 A/dm², 15 A/dm², 20 A/dm², 25A/dm², 30 A/dm², 35 A/dm², 40 A/dm², 45 A/dm², or 50 A/dm², where anyvalue may form an upper end point or a lower end point, as appropriate,depending on the separation between the cathode and anode. For example,an average current density ranging from about 15 A/dm² to about 30 A/dm²may be applied when the cathode and the anode is separated by about 3cm.

The pulsed current may have a duty cycle ranging from about 20% to about80%. The duty cycle may be about 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%,60%, 65%, 70%, 75%, or 80%, where any value may form an upper end pointor a lower end point, as appropriate. The pulsed current may have afrequency ranging from about 10 Hz to about 100 Hz. The frequency may beabout 10 Hz, 20 Hz, 30 Hz, 40 Hz, 50 Hz, 60 Hz, 70 Hz, 80 Hz, 90 Hz, or100 Hz, where any value may form an upper end point or a lower endpoint, as appropriate. For example, if the pulsed current has a dutycycle of about 40% and a frequency of about 25 Hz, the ON time is about16 milliseconds and the OFF time is about 24 milliseconds.

At block 306, a pH of the electrolyte solution is maintained at a targetpH or a target pH range. The target pH may be a pH ranging from about 2to about 4. The pH may be maintained at about 2, 2.2, 2.4, 2.6, 2.8,3.0, 3.2, 3.4, 3.6, 3.8, or 4.0, where any value may form an upper endpoint or a lower end point, as appropriate.

At block 308, a temperature of the electrolyte solution is maintained ata target temperature or a target temperature range. The targettemperature may be a temperature ranging from about 20° C. to about 60°C. The temperature may be about 20° C., 25° C., 30° C., 35° C., 40° C.,45° C., 50° C., 55° C., 60° C., 65° C., or 70° C., where any value mayform an upper end point or a lower end point, as appropriate.

In response to performing block 302, chromium is deposited on thesubstrate at block 310. In some examples, chromium and carbon areco-deposited on the substrate. Block 302 may be performed until achromium layer (e.g., a chromium coating) or a chromium-carbon layer(e.g., a chromium carbide coating) having a desired thickness (e.g., athickness greater than about 5 microns) is formed on the substrate. Thechromium layer having a thickness greater than about 5 microns may havehardness greater than about 800 HV.

FIG. 4 is an image of chrome plated substrates 410, 420, 430, 440, and450 formed by process 300 of FIG. 3, each plated at a different pH. Foreach chrome plated substrate 410, 420, 430, 440, and 450, the chromeplating parameters were as follows: the plating time was 1 hour, at atemperature of 35° C., and at a current density of 22 A/dm².

Chrome plated substrate 410 was plated at a pH of 1.5, resulting in achromium layer 412 having a thickness of 4 microns. Chrome platedsubstrate 420 was plated at a pH of 2.0, resulting in a chromium layer422 having a thickness of 5 microns. Chrome plated substrate 430 wasplated at a pH of 2.5, resulting in a chromium layer 412 having athickness of 20 microns. Chrome plated substrate 440 was plated at a pHof 3.0, resulting in a chromium layer 442 having a thickness of 30microns. A chrome plated substrate 450 was plated at a pH of 3.5,resulting in a chromium layer 452 having a thickness of 14 microns.

As illustrated by FIG. 4, any pH ranging from about 1.5 to about 3.5provides deposition of a chromium layer. A pH ranging from about 2 toabout 4 advantageously provides a thicker chromium layer that a pH thanis higher or lower. Further, a pH ranging from about 2.5 to about 3.0advantageously provides the thickest chromium layer.

FIG. 5 is an image of chrome plated substrates 510, 520, 530, 540, and550 formed by process 300 of FIG. 3, each plated at a differenttemperature. For each chrome plated substrate 510, 520, 530, 540, and550, the chrome plating was performed at a pH of 2.8.

Chrome plated substrate 510 was plated at a temperature of 30° C.,resulting in a chromium layer 512 having a thickness of 32 microns.Chrome plated substrate 520 was plated at a temperature of 40° C.,resulting in a chromium layer 522 having a thickness of 45 microns.Chrome plated substrate 530 was plated at a temperature of 50° C.,resulting in a chromium layer 532 having a thickness of 20 microns.Chrome plated substrate 540 was plated at a temperature of 60° C.,resulting in a chromium layer 542 having a thickness of 16 microns.Chrome plated substrate 550 was plated at a temperature of 70° C.,resulting in a chromium layer 552 having a thickness of 32 microns.

As illustrated by FIG. 5, any temperature ranging from about 30° C. toabout 70° C. provides deposition of a chromium layer. A temperatureranging from about 30° C. to about 40° C. advantageously provides thethickest chromium layer than a temperature that is higher or lower.

FIG. 6 is an image of chrome plated substrates 610, 620, 630, 640, and650 formed by process 300 of FIG. 3, each plated at a different currentdensity using direct current plating. For each chrome plated substrate610, 620, 630, 640, and 650, the chrome plating parameters were asfollows: the plating time was 1 hour, and the distance between thecathode and the anode was 3 cm.

Chrome plated substrate 610 was plated using a current density of 40A/dm², resulting in a chromium layer at a first location 612 having athickness of 60 microns, a chromium layer at a second location 614having a thickness of 60 microns, a chromium layer at a third location616 having a thickness of 60 microns, and an uncoated area 618surrounding chromium layer at locations 612, 614, and 616. Chrome platedsubstrate 620 was plated using a current density of 30 A/dm², resultingin a chromium layer at a first location 622 having a thickness of 30microns, a chromium layer at a second location 624 having a thickness of30 microns, a chromium layer at a third location 636 having a thicknessof 30 microns, and an uncoated area 628 surrounding chromium layer atlocations 622, 624, and 626 that is smaller than uncoated area 618.Chrome plated substrate 630 was plated using a current density of 20A/dm², resulting in a chromium layer at a first location 632 having athickness of 25 microns, a chromium layer at a second location 634having a thickness of 18 microns, and a chromium layer at a thirdlocation 636 having a thickness of 20 microns. Chrome plated substrate640 was plated using a current density of 10 A/dm², resulting in achromium layer at a first location 642 having a thickness of 2 microns,a chromium layer at a second location 644 having a thickness of 2microns, and a chromium layer at a third location 646 having a thicknessof 2 microns. Chrome plated substrate 650 was plated using a currentdensity of 5 A/dm², resulting in a chromium layer at a first location652 having a thickness of 0 microns, a chromium layer at a secondlocation 654 having a thickness of 0 microns, and a chromium layer at athird location 656 having a thickness of 0 microns.

As illustrated by FIG. 6, any current density ranging from about 5 A/dm²to about 40 A/dm² provides deposition of a chromium layer when thedistance between the cathode and the anode is about 3 cm. A currentdensity ranging from about 10 A/dm² to about 30 A/dm² advantageouslyprovides a chromium layer that is thick and at the same time uniform, asa current density of 5 A/dm² does not provide chromium layer depositionand a current density of 40 A/dm² provides a less uniform chromium layerdeposition as shown by uncoated area 618. Further, a current density ofabout 20 A/dm² may advantageously provide the thickest chromium layerwhile still coating the whole substrate surface, and also minimizegeneration of chlorine gas.

FIG. 7 is an image of chrome plated substrates 710, 720, 730, and 740formed by process 300 of FIG. 3, each plated at a different averagecurrent density using pulsed current plating. For each chrome platedsubstrate 710, 720, 730, and 740, the chrome plating parameters were asfollows: the plating time was 1 hour, and the distance between thecathode and the anode was 3 cm, and the duty cycle was 40%.

Chrome plated substrate 710 was plated using an average current densityof 40 A/dm², resulting in a chromium layer at a first location 712having a thickness of 62 microns, a chromium layer at a second location714 having a thickness of 62 microns, a chromium layer at a thirdlocation 716 having a thickness of 85 microns, and an uncoated area 718around chromium layer at locations 712, 714, and 716. Chrome platedsubstrate 720 was plated using an average current density of 30 A/dm²,resulting in a chromium layer at a first location 722 having a thicknessof 38 microns, a chromium layer at a second location 724 having athickness of 50 microns, and a chromium layer at a third location 736having a thickness of 60 microns. Chrome plated substrate 730 was platedusing an average current density of 20 A/dm², resulting in a chromiumlayer at a first location 732 having a thickness of 10 microns, achromium layer at a second location 734 having a thickness of 15microns, and a chromium layer at a third location 736 having a thicknessof 20 microns. Chrome plated substrate 740 was plated using an averagecurrent density of 10 A/dm², resulting in a chromium layer at a firstlocation 742 having a thickness of 0 microns, a chromium layer at asecond location 744 having a thickness of 0 microns, and a chromiumlayer at a third location 746 having a thickness of 0 microns.

As illustrated by FIG. 7, any average current density ranging from about20 A/dm² to about 40 A/dm² provides deposition of a chromium layer whenthe distance between the cathode and the anode is about 3 cm. A currentdensity ranging from about 20 A/dm² to about 30 A/dm² advantageouslyprovides a chromium layer that is thick and at the same time uniform, asan average current density of 10 A/dm² does not provide chromium layerdeposition and an average current density of 40 A/dm² provides a lessuniform chromium layer deposition as shown by uncoated area 718.Further, an average current density of about 20 A/dm² may advantageouslyprovide the thickest chromium layer while still coating the wholesubstrate surface, and also minimize generation of chlorine gas.

FIG. 8 is a graph showing thickness of chromium layers formed by process300 of FIG. 3 using pulsed current plating at different pulsefrequencies and duty cycles. Pulse plating was carried out at thefrequencies of 10 Hz, 25 Hz, 50 Hz, and 100 Hz, and at duty cycles of10%, 20%, 40%, and 80% for each frequency.

As illustrated by FIG. 8, any frequency ranging from about 10 Hz to 100Hz, and any duty cycle ranging from about 10% and 80% providesdeposition of a chromium layer. A duty cycle of about 40% at a frequencyof about 25 Hz, which corresponds to an ON time of 16 milliseconds andan OFF time of about 24 milliseconds, advantageously provides thethickest chromium layer having a thickness of about 16 microns.

FIG. 9 is an image of a chrome plated substrate 900 formed by process300 of FIG. 3 using pulsed current, while FIG. 10 is an image of achrome plated substrate 1000 formed by process 300 of FIG. 3 usingdirect current. As shown in FIG. 9, chrome plated substrate 900 has achromium layer 902 that is uniformly and compactly deposited. FIG. 11 isa scanning electron microscopy (SEM) image of a cross-section of chromeplated substrate 900, showing that chromium layer 902 is well adhered toa substrate 910 and compact. As shown in FIG. 10, chrome platedsubstrate 1000 has non-adherent areas 1004 and has a chromium layer 1002that is less compactly deposited. FIG. 12 is a SEM image of across-section of chrome plated substrate 1000, showing that chromiumlayer 1002 has parts 1006 that are less-adherent to a substrate 1010 andless compact.

As illustrated by FIGS. 9-12, for thick hard chromium coating (e.g.,coating thickness of greater than about 30 microns), chrome platingusing pulsed current advantageously provides more adherent and morecompact chromium deposits compared to chrome plating using directcurrent.

FIG. 13 is an image of a chrome plated substrate such as a chrome platedHull cell panel 1300 formed by the process of FIG. 3 using anelectrolyte solution prepared by dissolving chromium (III) sulfate andan oxalate compound but not dissolving alkali metal sulfate. The chromeplating was performed in a Hull cell at 5 Amperes for 10 minutes. Chromeplated Hull cell panel 1300 shows only about 50% coverage, with an area1302 covered by a chromium layer and an area 1304 not covered bychromium.

FIG. 14 is an image of a chrome plated substrate such as a chrome platedHull cell panel 1400 formed by the process of FIG. 3 using theelectrolyte solution prepared by the process of FIG. 1 without the stepof dissolving the surfactant. The electrolyte solution was prepared bydissolving chromium (III) chloride, an oxalate compound, and also analkali metal sulfate such as sodium sulfate. Chrome plated Hull cellpanel 1400 shows more than 80% coverage, with an area 1402 covered by achromium layer and an area 1404 not covered by chromium. The electrolytesolution prepared from chromium (III) chloride and sodium sulfateprovided improved coverage compared to the electrolyte solution preparedfrom chromium (III) sulfate and no sodium sulfate used for chrome platedHull cell panel 1300 in FIG. 13.

FIG. 15 is an image of a chrome plated substrate such as a chrome platedHull cell panel 1500 formed by the process of FIG. 3 using theelectrolyte solution prepared by the process of FIG. 1 including thestep of dissolving the surfactant. The electrolyte solution was preparedby dissolving chromium (III) chloride, an oxalate compound, an alkalimetal sulfate such as sodium sulfate, and the surfactant such as sodiumlauryl sulfate. Chrome plated Hull cell panel 1500 shows more than 80%coverage, with an area 1502 covered by a chromium layer and an area 1504not covered by chromium.

As illustrated by FIGS. 13-15, an electrolyte solution in which analkali metal sulfate such as sodium sulfate is dissolved advantageouslyprovides improved chrome plating, with a significantly higher percentcoverage of the substrate. The alkali metal sulfate may, for example,provide increased conductivity to the electrolyte solution, resulting inan improved chromium layer deposition.

FIGS. 16A-B are SEM images of a part of chrome plated Hull cell panel1600 of FIG. 14. A 1 cm² portion was cut out from the middle of chromeplated Hull cell panel 1400 and SEM images were taken—FIG. 16A is an SEMimage at 1000× magnification, and FIG. 16B is an SEM image at 2500×magnification. Chrome plated Hull cell panel 1400 formed using theelectrolyte solution without the surfactant showed many pits, appearingas black spots on the SEM images of FIGS. 16A and 16B.

FIGS. 17A-B are SEM images of a part of chrome plated Hull cell panel1500 of FIG. 15. A 1 cm² portion was cut out from the middle of chromeplated Hull cell panel 1500 and SEM images were taken—FIG. 17A is an SEMimage at 1000× magnification, and FIG. 17B is an SEM image at 2500×magnification. Chrome plated Hull cell panel 1500 formed using theelectrolyte solution with the surfactant did not show pits, as there areno black spots on the SEM images of FIGS. 17A-B compared to FIGS. 16A-B.

As illustrated by FIGS. 16A-B and 17A-B, including the surfactant in theelectrolyte solution advantageously has the effect of reducing pitting.The surfactant may function as a wetting agent that reduces the surfacetension, and may reduce the generation of gas (e.g., chlorine gas,hydrogen gas, etc.) during chrome plating. The generation of gas mayform pores in the chromium that is deposited, which may appear as pitswhen the gas generation is in excess. The surfactant, by reducing gasevolution, prevents or reduces such pitting during chrome plating.

FIG. 18 is an SEM image of chrome deposits 1802 on a chrome platedsubstrate formed by the process of FIG. 3. As shown in FIG. 18, chromedeposits 1802 have an amorphous morphology. A Vickers indent test wasperformed on chrome deposits 1802 at location 1804, which revealed thatchrome deposits 1802 had a hardness of about 1100 HV at a 100 g load.

FIG. 19 is an image of a chrome plated substrates 1900 formed by theprocess of FIG. 3 that have been bent to show resilience of chromiumlayers 1902 to bending. Even when chrome plated substrates 1900 arebent, chromium layers 1902 do not come off, revealing that chromiumlayer 1902 are strongly adherent to the underlying substrate.

FIG. 20 is an image of a chrome plated substrate 2000 formed by theprocess of FIG. 3 on which abrasion testing has been performed todetermine wear resistance of a chromium layer 2002. A CS 10 wheel under1000 g load was used, resulted in a wear index of about 0.013 to about0.021 at a tested area 2004 of chromium layer 2002. A further test wasperformed using a CS 17 wheel under 1000 g load, which resulted in awear index of about 0.015 to 0.025. The abrasion testing revealed thatthe wear property is similar to hard chromium layers formed by chromeplating using hexavalent chromium.

Although there have been some successes at implementing the use oftrivalent chromium baths for thin, decorative chrome plating,conventional chrome plating processes that use trivalent chromium bathswere unsuitable for thicker, hard chrome plating. Moreover, eventrivalent chromium baths used for decorative chrome plating oftencontained boric acid as a buffering agent. Further, conventional chromeplating processes that use trivalent chromium risked the trivalentchromium being oxidized to hexavalent chromium at the anode.

Advantageously, chrome plating according to process 300 provides hardchromium layers that may be at least as structurally robust, reliable,adherent, and wear resistant as chrome plating using hexavalentchromium, while avoiding the use of chemicals such as hexavalentchromium and boric acid. Further, oxidation of trivalent chromium tohexavalent chromium, generation of toxic gas byproducts, and theproduction of further undesirable byproducts are avoided orsignificantly reduced.

Embodiments described above illustrate but do not limit the invention.It should also be understood that numerous modifications and variationsare possible in accordance with the principles of the presentdisclosure. Accordingly, the scope of the invention is defined only bythe following claims.

What is claimed is:
 1. A method for chrome plating a substrate using anelectrolyte solution, the method comprising: dissolving in an aqueousmedium a trivalent chromium salt in an amount ranging from about 0.1 molto about 0.9 mol per liter of the electrolyte solution; dissolving anoxalate compound in an amount ranging from about 0.1 mol to about 3.0mol per liter of the electrolyte solution; dissolving a metal salt in anamount ranging from about 0.1 mol to about 4.0 mol per liter of theelectrolyte solution, an alkali metal sulfate in an amount ranging fromabout 0.1 mol to about 2.0 mol per liter of the electrolyte solution,and an alkali metal halide in an amount ranging from about 0.1 mol toabout 0.5 mol per liter of the electrolyte solution; and passing acurrent between a cathode and an anode through the electrolyte solutionto deposit chromium on the substrate.
 2. The method of claim 1, whereinthe step of dissolving the trivalent chromium salt comprises dissolvingchromium (III) chloride and/or chromium (III) sulfate.
 3. The method ofclaim 1, wherein the step of dissolving the oxalate compound comprisesdissolving sodium oxalate in an amount ranging from about 0.1 mol toabout 1.0 mol per liter of the electrolyte solution, potassium oxalatein an amount ranging from about 0.1 mol to about 1.0 mol per liter ofthe electrolyte solution, and/or oxalic acid in an amount ranging fromabout 0.1 mol to about 3.0 mol per liter of the electrolyte solution. 4.The method of claim 1, wherein: dissolving the metal salt comprisesdissolving aluminum sulfate in an amount ranging from about 0.1 mol toabout 0.4 mol per liter of the electrolyte solution and/or aluminumchloride in an amount ranging from about 0.1 mol to about 4.0 mol perliter of the electrolyte solution; dissolving the alkali metal sulfatecomprises dissolving sodium sulfate and/or potassium sulfate; anddissolving the alkali metal halide comprises dissolving sodium fluorideand/or potassium fluoride.
 5. The method of claim 1, wherein thedissolving the trivalent chromium salt, the oxalate compound, the metalsalt, the alkali metal sulfate, and the alkali metal halide is performedin the following order: (1) dissolving the trivalent chromium salt andthe oxalate compound; (2) dissolving the metal salt; (3) dissolving thealkali metal sulfate; and (4) dissolving the alkali metal halide.
 6. Themethod of claim 1, wherein the step of dissolving the oxalate compoundcomprises stirring the oxalate compound at a temperature ranging fromabout 70° C. to about 80° C. for a time ranging from about 1 hour toabout 3 hours.
 7. The method of claim 1, further comprising adjustingthe pH of the electrolyte solution to a pH ranging from about 2 to about4.
 8. The method of claim 1, further comprising adding sodium laurylsulfate and/or potassium lauryl sulfate in an amount ranging from about0.1 g to about 1 g per liter of the electrolyte solution.
 9. The methodof claim 1, further comprising adding sodium bromide and/or potassiumbromide in an amount ranging from about 0.1 g to about 1 g per liter ofthe electrolyte solution.
 10. The electrolyte solution prepared by themethod of claim
 1. 11. The method of claim 1, further comprisingmaintaining the electrolyte solution at a pH ranging from about 2 toabout
 4. 12. The method of claim 1, further comprising maintaining theelectrolyte solution at a temperature ranging from about 30° C. to about40° C. during the step of passing the current.
 13. The method of claim1, wherein the step of passing the current is performed using acarbonaceous anode, a platinum anode, or a platinized titanium anode,and wherein the trivalent chromium salt comprises chromium (III)sulfate.
 14. The method of claim 1, wherein the step of passing thecurrent is performed using a carbonaceous anode, and wherein thetrivalent chromium salt comprises chromium (III) chloride.
 15. Themethod of claim 1, wherein the step of passing the current comprisesapplying a pulsed current or a direct current having a current densityranging from about 5 A/dm² to about 50 A/dm².
 16. The method of claim 1,wherein the step of the passing the current comprises applying a pulsedcurrent having a duty cycle ranging from about 20% to about 80%.
 17. Themethod of claim 1, wherein the step of passing the current is performeduntil a chromium layer having a thickness greater than about 5 micronsand hardness greater than about 800 HV is formed on the substrate. 18.The method of claim 1, wherein the step of passing the current todeposit chromium on the substrate comprises passing the current todeposit chromium on a steel substrate, a copper substrate, a nickelsubstrate, a copper-coated substrate, or a nickel-coated substrate. 19.The method of claim 1, further comprising responsive to the step ofpassing the current, depositing chromium on the substrate orco-depositing chromium and carbon on the substrate.
 20. A method forpreparing an electrolyte solution for chrome plating, the methodcomprising: providing trivalent chromium by dissolving a trivalentchromium salt; forming complexes of oxalate and trivalent chromium bydissolving an oxalate compound; buffering the electrolyte solution bydissolving a metal salt; increasing the conductivity by dissolving analkali metal sulfate; and increasing the wetting property of theelectrolyte solution by dissolving alkali metal halide.